The present invention relates to controlling an electric motor. In particular, the present invention relates to an apparatus and method for controlling an electric motor so as to reduce torque ripple.
There are many known types of power assist steering systems for automotive vehicles. Some systems provide steering assist by using hydraulic power, some by using electric power, and others by a combination of hydraulic and electric power.
An electric assist steering system includes an electric motor drivingly connected to steerable vehicle wheels. When energized, the electric motor assists the steering movement of the steerable wheels. The electric assist motor is controlled in response to steering torque applied to the steering wheel.
A typical control arrangement for an electric motor includes a motor controller that provides a current command signal for energizing the electric motor. A current sensor senses the electric current in the electric motor and provides a signal indicative of the sensed current. An error signal is derived from the current command signal and the sensed current signal. A voltage command signal is then determined based on the error signal. The voltage command signal is provided to a switching circuit, which typically includes a pulse width modulation inverter. The pulse width modulation inverter, in turn, drives the electric motor in response to the determined voltage command signal.
Examples of motor control arrangements for reducing torque ripple are disclosed in U.S. Pat. Nos. 4,611,158, 5,223,775, 5,350,989, 5,616,999, 5,821,714, and 5,821,725.
One aspect of the present invention provides an apparatus for controlling an electric motor having a plurality of motor phases. The apparatus includes a motor controller that controls energization of each phase of the plurality of motor phases. A compensation circuit is associated with each phase of the plurality of motor phases. The compensation circuit of one phase of the plurality of motor phases adjusts a control parameter of another phase of the plurality of motor phases an amount functionally related to an electrical characteristic of the one phase in response to determining a diminished operating characteristic of the one phase.
Another aspect of the present invention provides an apparatus for controlling an electric motor that includes an electric motor having a plurality of motor phases. A motor controller provides a current command signal for controlling energization of each phase. A current sensor senses electric current of each respective phase of the electric motor. The system also includes a plurality of current controllers, each of which controls energization of an associated phase based on the current command signal for the associated phase and the sensed electric current of the associated phase. A compensation circuit of the associated phase provides a compensation signal for adjusting the current command signal of another phase in response to determining a diminished operating characteristic of the associated phase.
Another aspect of the present invention provides a method for controlling operation of an electric motor having a plurality of motor phases. The method includes the steps of providing a command signal for controlling energization of each of the plurality of motor phases and determining a diminished operating characteristic for an energized phase of the plurality of motor phases. A control parameter of another phase is adjusted an amount functionally related to an electric characteristic of the energized phase of the plurality of motor phases in response to determining the diminished operating characteristic.